The present invention relates to digital imaging, and more particularly, to a system for and method of reducing x-ray exposure time of normal examination procedures, i.e. of those procedures that do not use magnification geometry, performed with digital mammographic equipment.
The focal spot of a radiography system is the linear dimension of the ortho-normal projection on the image plane of the region of the x-ray source, that is the area of the anode, that radiates photons. Typically, the focal spot of currently available, film-based mammography systems has two nominal sizes: 0.3 mm, for use in normal examinations; and 0.1 mm, for use in magnification procedures. The reason for maintaining a relatively small focal spot in normal mammographic examinations performed with film-based equipment is to prevent further loss of image resolution in the range of spatial frequencies of diagnostic interest (typically between 1 and 5 lp/mm), such a loss being beyond the loss of resolution caused by the finite thickness of the scintillating material of the film/screen detector. On the other hand, it is well known that a small focal spot size results in long exposure times, because the photon flux from the x-ray source is a strong function of the focal spot size; thus a small focal spot results in low x-ray flux.
A long exposure time, in turn, results in various negative effects, the most important of which is an increased probability that the breast under examination could move during the exposure, so that its image is smeared and therefore diagnostically unacceptable. Another reason to keep the exposure time as short as possible is to minimize the time during which the breast of the patient is painfully compressed. This approach, however, is relevant only when the reduction of the exposure time results in a significant reduction of the time during which the breast is under compression. Current generation, digital mammography equipment has critically adopted the same small focal spot size used in film-based systems. This, in turn, necessitates a relatively long exposure time, that is in the range 0.5 to 4.0 seconds depending on the beam quality and the breast thickness, whilst it is common experience that when the duration of the exposure exceeds 2 seconds, the probability of breast motion becomes significant. It is desirable to substantially reduce the x-ray exposure time of normal breast examination (i.e. the term “normal examination” is intended to mean examination that does not use magnification geometry where the image is projected to be larger than the original object) using digital mammography systems.